General introduction to chronobiology.

Rhythmicity is part of our life. All living organisms exhibit in some way or another a form of rhythmicity. Some of these rhythms have a geophysical counterpart in the environment. These are the so-called biological rhythms. This paper gives an introduction to the circadian rhythms, i.e., those rhythms with a period length of about one day. General characteristics and properties are described as well as the control and disturbances of these rhythms.